Gram Research analysis of a 2026 study found that harbour porpoises in Portuguese waters mainly eat small coastal fish, especially common dragonet, pouting, and mullet. Based on 157 stomach samples from 2003 to 2020, researchers also found that fishing grounds overlap heavily with porpoise feeding areas, which may help explain high bycatch in northern Portugal.
A new study published in Marine Environmental Research looked at what harbour porpoises in Portuguese waters eat. Gram Research analysis of this work shows the team combined stomach sample analysis with chemical tests on animal tissues, covering the years 2003 to 2020. The main foods were common dragonet, pouting, and mullet, all small fish that live near the coast. The chemical results matched the stomach findings, suggesting pouting has been a steady food source for these animals for many years. The researchers also found that fishing areas overlap heavily with porpoise feeding areas. This overlap may help explain why many porpoises are accidentally caught in fishing nets in northern Portugal. Portugal recently classified this porpoise population as Critically Endangered, so the findings may help guide future protection plans.
Key Statistics
A 2026 study analyzing 157 harbour porpoise stomach contents from 2003 to 2020 found that common dragonet, Trisopterus species, and Chelon species were the main prey of porpoises in Portuguese waters.
A 2026 Marine Environmental Research study found that chemical tests of porpoise tissue matched stomach findings, with Trisopterus species, including pouting, identified as a long-term food source in Portugal.
A 2026 study of Portuguese harbour porpoises reported a marked overlap between fishing grounds and porpoise feeding areas, which may explain high bycatch rates in northern Portugal.
The Quick Take
- What they studied: What do harbour porpoises in Portuguese waters eat, and does their diet change over time and season? Knowing this can help protect them.
- Who participated: The study examined 157 stomach contents from harbour porpoises found in Portuguese waters between 2003 and 2020. Tissue samples were also tested with chemical methods. The summary does not state the total number of animals.
- Key finding: Common dragonet, pouting, and mullet were the main prey. Chemical results agreed with the stomach findings, which makes the conclusions more trustworthy. The study also found that fishing grounds overlap closely with porpoise feeding areas.
- What it means for you: This research is mainly useful for fisheries managers and conservation groups. It does not give everyday people a personal health or diet rule. Anyone who buys seafood can support fishing methods that reduce accidental catches of marine mammals.
The Research Details
Researchers used two main methods to learn what porpoises eat. The first was stomach content analysis, which means examining the remains of food found in the stomachs of porpoises that had died. This shows what the animal ate most recently, and it provides a direct look at specific prey species.
The second method was stable isotope analysis. Isotopes are slightly different forms of chemical elements that build up in an animal’s body tissues depending on what it eats. By testing muscle and liver tissue, scientists can estimate what an animal has eaten over a longer period, often weeks or months. They can also estimate whether an animal feeds near the coast or far out at sea.
By combining both methods, the team could check whether the two approaches agreed. They also compared results across different years and seasons. This helped them see whether the porpoises’ diet changed over time or with the seasons.
Stomach samples show exactly which fish were eaten, but they only capture a snapshot of the last meal. Chemical tests reveal a broader pattern of eating over time. When both methods point to the same prey, that agreement gives researchers more confidence in the conclusions. Using two methods together also reduces the chance that one unusual sample misleads the results.
The study covers almost two decades, which strengthens its findings because seasonal and yearly patterns are less likely to be accidental. Using two independent methods that produced consistent results is a strong sign of reliability. Still, the summary does not list the full number of animals or the number of tissue samples, so readers cannot judge the exact size of the sample. The paper was published in a respected, peer-reviewed environmental science journal.
What the Results Show
The stomach samples showed that common dragonet, Trisopterus species, and mullet were the main foods of harbour porpoises in Portuguese waters. Common dragonet is a small bottom-dwelling fish, while Trisopterus species include pouting, a fish that lives in shallow coastal waters. These prey species are typically found close to shore.
Chemical results suggested that porpoises feed mostly in coastal areas rather than in deep offshore waters. Muscle tissue chemistry pointed to Trisopterus as the most likely main food. This matches the stomach findings and suggests a consistent pattern.
Liver tissue results showed some variation in diet. The researchers explain that these differences depend on how the body processes food, which can shift the chemical signals. This means liver results need careful interpretation and are less direct than muscle results.
The diet also appeared to change across years and seasons. The researchers suggest porpoises may switch to different prey depending on what is available, where they travel, and what their bodies need at different times of year. Overall, Trisopterus appears to have been a long-term food source in Portugal.
The study found a strong overlap between fishing grounds and porpoise feeding areas. This suggests that porpoises and fishing boats often use the same waters for the same types of fish. That overlap may explain why porpoises are frequently caught accidentally in fishing gear in northern Portugal. Reducing this overlap could be one practical step toward protecting the population.
Earlier research on harbour porpoises in European waters has also found that they tend to prefer small, bottom-living or coastal fish. This study agrees with that broader pattern. The long time span and the combined use of two methods add new detail specific to Portuguese waters, which had less data available before.
The study does not clearly state how many porpoises were included in the chemical tests, so it is hard to judge how broadly the results apply. Stomach samples come from animals that were found dead, which may not represent the whole population. Chemical signals in the liver are affected by how the body processes food, so those results are less certain. The study also shows an overlap between fishing and feeding areas, but it does not directly prove that this overlap causes the bycatch. Finally, the findings come from one country’s waters and may not apply to porpoises elsewhere.
The Bottom Line
Conservation groups and fisheries managers should consider the overlap between fishing grounds and porpoise feeding areas when planning protections. Confidence is moderately high that small coastal fish such as pouting and common dragonet are important foods for these porpoises, because two different methods agreed. Confidence is lower that specific fishing changes would fully solve the bycatch problem, since more monitoring is needed to measure the effect.
Fisheries managers, marine conservation teams, policymakers, and researchers working on Portuguese or Iberian marine life should pay close attention. Seafood buyers who care about marine wildlife may also find the results useful. People who are not interested in marine conservation or fisheries management are unlikely to need to act on these findings directly.
Changes to fishing practices or protected zones usually take several years to plan, approve, and carry out. Scientists would then need additional years of monitoring to see whether porpoise bycatch rates drop. Individuals will not see direct changes in the porpoise population from personal actions in the short term.
Frequently Asked Questions
What do harbour porpoises eat in Portuguese waters?
Harbour porpoises in Portuguese waters mainly eat small coastal fish. A 2026 study of 157 stomach samples identified common dragonet, Trisopterus species such as pouting, and mullet as the main prey. Their diet may shift with prey availability and season.
Why are harbour porpoises caught in fishing nets in Portugal?
Research reviewed by Gram suggests that porpoises and fishing boats often use the same waters for the same fish. This marked overlap between fishing grounds and feeding areas may explain high bycatch rates, especially in northern Portugal.
How do scientists figure out what marine mammals eat?
Scientists use stomach content analysis to identify recent meals and stable isotope analysis to estimate diet over weeks or months. Using both methods together helps confirm results. In this Portuguese porpoise study, the two methods produced consistent findings.
Why is the harbour porpoise considered endangered in Portugal?
Portugal recently classified the harbour porpoise population as Critically Endangered at the national level. This status highlights the urgent need for a dedicated conservation action plan, which research on feeding ecology can help guide.
Want to Apply This Research?
- Log how many seafood meals you eat each week and note whether the label shows a catch method or origin. Track the number of meals that include wild-caught fish from certified sustainable fisheries. Set a simple weekly goal, such as choosing one certified option per week.
- When buying fish, look for labels from recognized sustainability programs and ask fishmongers about how the fish was caught. Choose options that use bycatch-reducing gear when available. Use the app to note the choice each time you shop so you can see your progress.
- Review your seafood logs monthly and compare the number of certified sustainable choices over time. Update your goals each season, since fish availability changes throughout the year. Consider checking for updates from conservation groups or research summaries about Portuguese marine life every few months.
This article summarizes a wildlife research study and is intended for general information only. It does not provide medical, dietary, or legal advice. Readers interested in marine conservation policy or fishing regulations should consult official government agencies or qualified marine scientists.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.